1. Field of the Invention
Embodiments of the present invention relate generally to methods of manufacturing an alternating phase-shift mask used for fabricating semiconductor devices. More specifically, embodiments of the invention relate to methods of manufacturing an extreme ultra-violet lithography (EUVL) alternating phase-shift mask.
A claim of priority is made to Korean Patent Application No. 2005-0032756, filed on Apr. 20, 2005, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of Related Art
Photolithography processes are commonly used to form minute patterns in electronic devices such as integrated circuits. In general, the minimum size of a pattern that can be formed by a photolithography process is limited by the resolution of photolithography equipment used to carry out the process. Where the desired critical dimension of a pattern approaches the resolution of the photolithography equipment, a proximity effect may occur. Briefly, the proximity effect includes undesirable structural interactions between adjacent features formed by the pattern. In the case of photolithography processes, the proximity effect generally results from electron scattering in an irradiated resist layer where the pattern is formed.
One proposed method for addressing the proximity effect in photolithography processes using a light source such as a krypton fluoride (KrF) or argon fluoride (ArF) laser is to shift the light source's phase using a transmitting phase-shift mask. Shifting the light source's phase introduces destructive interference which can prevent some of the electron scattering. One way to form the transmitting phase-shift mask is by etching a phase-shift region in a quartz substrate so that the quartz substrate will reflect the light source with respective phases of 0° and 180°.
One shortcoming of the above method is that light passing through the phase-shift region can be scattered, for example, by the sidewalls of the etched region. As a result, the intensity of light passing through the phase-shift region may be lower than the intensity of light passing through other portions of the quartz substrate. Due to this light intensity difference, a critical dimension (CD) difference (ΔCD) may arise between adjacent patterns transferred on a wafer. In addition, when the phase-shift deviates from 180°, a ΔCD reversal, which is also called an X-phenomenon, may occur. To address the X-phenomenon, an undercut is generally formed in the etched phase-shift region using an isotropic wet etching process to prevent light loss from occurring.
However, in next-generation EUVL exposure technology, because the absorbency of EUV light sources having a short wavelength is too high when transmitting masks are used, reflective masks are used instead of transmitting masks. To maximize the reflectivity of EUV with a wavelength of 13.5 nm, a reflective mask includes a reflective layer including two types of material alternately stacked a plurality of times. For example the reflective layer could comprise 40 pairs of alternately stacked molybdenum (Mo) and silicon (Si) layers with a chromium (Cr) shielding layer pattern formed thereon. Similar to the transmitting masks, the intensity of light reflected by a phase-shifting region of a reflecting phase-shift mask is generally lower than the intensity of light reflected by other portions of the reflecting phase-shift mask. Accordingly, the reflecting phase-shift mask also suffers from the ΔCD problem. Unfortunately, however, the ΔCD cannot be addressed by the same method used to reduce ΔCD in transmitting phase-shift masks.
Several methods have been proposed for reducing the ΔCD or X-phenomenon created by EUVL alternating phase-shift masks. Some of these methods are disclosed, for example, in the following two documents: “EUVL Alternating Phase-shift Mask Imaging Evaluation”, Pei-Yang Yan et al., Proc. Of SPIE Vol. 4889; and, “Phase-shift Mask in EUV Lithography”, Minoru Sugawara et al., SPIE Vol. 5037.
Unfortunately, all of the proposed methods are difficult and complicated to implement and therefore highly impractical.